Kryptografia - pytania

Question

Witam. Mam szerego pytan dotyczących kryptografii na które nie udało mi się niestety znaleźć odpowiedź. Czy moglibyscie rzucić poradą lub podać mi źródła w które pomogłyby mi na nie odpowiedzieć? Z góry dziękuję.

1. The Vernam encrypts purely by substitution, without transposition? Yes or No?

2. Suppose we replace the XOR operation in the Vernam with the boolean OR operation and use a random key. If the probability of the plaintext bit being 1 is 0.3 (i.e. Pr[M=1] =0.3), then what is the probability of the ciphertext bit being 1 (i.e Pr[C=1])?

3. The recurrence period of any L-bit LFSR is 2L-1 ? Yes or No?

4. To improve the security of the bits produced by LFSR, what can we do? Choose all the correct ones.

a) Use multiple LFSRs to produce the bits.

b) Use a random state as the initial state.

c) Use nonlinear functions when combining outputs from multiple LFSRs.

d) Increase the size of the LFSR.

5. Given the following LFSR setting, what will be the state after 4 cycles (your answer must contain only 0 and 1, no space or other characters)?

6. Which one of the following statements about the IV (initialization vector) is correct? 

a) An IV is a value derived from the secret key

b) An IV is the starting state of an LFSR

c) An IV is XORed with the plaintext during encryption.

d) An IV needs to be long enough to be secure.

 

7. Suppose you have a single character plaintext, that is "A" (uppercase A, ASCII encoded). You encrypted it using a stream cipher. During transmission, the last bit (the rightmost bit e.g. 00000001 the last bit is 1) in the ciphertext is flipped. What will the decryption result be (a single character in your answer, no space)?